This invention relates to a driving wheel slip control system for vehicles, and more particularly to a driving wheel slip control system for vehicles in which is installed an internal combustion engine capable of changing the valve lift characteristic of inlet valves and/or exhaust valves.
As recognized in general, a driving wheel of an automotive vehicle undergoes a slip when the vehicle is started to run or when it is accelerated, if the driving force of the diving wheel surpasses a frictional force developed between the tire of the driving wheel and the road surface [=the coefficient of friction between the tire and the road surface.times.load of the vehicle weight on the driving wheel (wheel load)]. Conventionally, a driving wheel slip control system has been proposed, e.g., by Japanese Provisional Patent Publication (Kokai) No. 58-8436, in which the degree of the slip is detected by a difference .DELTA.V between the driving wheel speed and the trailing wheel speed, and when an excessive slip state in which the difference .DELTA.V is great is detected, the amount of fuel supplied to the engine is reduced by fuel cut or by some other means, to thereby reduce the output torque (driving force) of the engine. In this driving wheel slip control system, as the degree of excessive slip is increased, fuel cut is effected to more cylinders.
In the meanwhile, a valve lift characteristic control system has been proposed. e.g., by Japanese Patent Publication (Kokoku) No. 49-33289, in which the valve lift characteristic of inlet valves and/or exhaust valves can be switched between a low speed valve lift characteristic suitable for a low engine rotational speed region and a high speed valve lift characteristic suitable for a high engine rotational speed region, and engine operating conditions (e.g. engine rotational speed, and intake pressure) are detected to thereby select one of the valve lift characteristics suitable for the detected engine operating conditions.
The output torque of an internal combustion engine which is capable of switching the valve lift characteristic changes relative to the engine rotational speed in dependence on the valve lift characteristic, e.g. as shown in FIG. 14. More specifically, on the lower engine rotational speed side (Ne&lt;Nev), the output torque is larger when the low speed valve lift characteristic is selected than when the high speed valve lift characteristic, is selected, while on the higher engine rotational speed side (Ne&gt;Nev), the output torque is larger when the high speed valve lift characteristic is selected than when the low speed valve lift characteristic is selected. Thus, when the valve lift characteristic is changed over, the output torque of the engine changes. According to the conventional driving wheel slip control system, if the changeover is carried out during the driving wheel slip control, the slip state (magnitude of slip) of driving wheels is changed due to change in the output torque of the engine resulting from the changeover of the valve lift characteristic. The control system responds to the change in the slip state, and then the slip control suitable for the output torque of the engine is resumed. Therefore, it has been impossible to avoid change in the slip state of driving wheels resulting from changeover of the valve lift characteristic, which results in degraded driveability of the vehicle.